Switched mode power supplies can be used to create a direct current (DC) voltage from a DC or an alternating current (AC) input voltage by switching current through an energy storage element such as a transformer. The duty cycle of the switching is controlled to regulate the output voltage to a desired level. The secondary side of the transformer is used to deliver power to a load at a regulated voltage. Typically, the switched mode power supply delivers power to an output capacitor and the load through a rectifier, which prevents reverse current flow when the power supply conducts current through the primary winding of the transformer.
The rectifier can take two forms. A passive rectifier, such as a diode, can be placed in series with the secondary winding to prevent reverse current flow. However the diode cannot properly prevent reverse current flow if the output power supply voltage exceeds the breakdown voltage of the diode. Moreover the diode causes a forward voltage drop when conductive, decreasing the efficiency of the converter. To solve these problems, another form of rectifier known as a synchronous rectifier is often used. A synchronous rectifier includes an active switch, typically an N-channel metal-oxide-semiconductor field effect transistor (MOSFET), connected in series with the secondary winding along with a controller that makes the transistor conductive at the appropriate time. Because the transistor can be biased fully on, synchronous rectifiers are generally more efficient than passive rectifiers.
Synchronous rectifiers compare the drain voltage to various thresholds to determine when the make the synchronous rectifier transistor conductive and non-conductive. To reduce dead time and achieve high efficiency, a higher turn-off threshold voltage is required. If stray inductance caused by the MOSFET package and the printed circuit board (PCB) pattern is large, a positive turn-off threshold shows much higher system efficiency with a small dead time. However, the positive turn-off threshold may induce late turn-off during a transient condition and lead to inversion currents and drain spiking. To prevent the late turn-off during transient conditions, the off-time threshold can be set to around zero volts or even to a negative value. However a zero or negative turn off threshold decreases system efficiency itself. Thus it has been difficult to simultaneously achieve both a small dead time while avoiding inversion currents and drain spiking during synchronous rectifier turn off.
The use of the same reference symbols in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.